MICRF003 Просмотр технического описания (PDF) - Micrel

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MICRF003

QwikRadio™ 900 MHz UHF Receiver

Micrel 

MICRF003

I/O Pin Interface Circuitry

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3. CAGC Pin

Micrel

Interface circuitry for the various I/O pins of the MICRF003

is shown in Figures 1 through 6. Specific information

regarding each of these circuits is discussed in the following

sub-paragraphs. Not shown are ESD protection diodes

which are applied to all input and output pins.

Figure 3 illustrates the CAGC pin interface circuit. The

AGC control voltage is developed as an integrated current

into a capacitor CAGC. The attack (pulldown) current is

nominally 15µA, while the decay (pullup) current is a 1/10th

scaling of this, nominally 1.5µA, so the attack/decay

timeconstant ratio is fixed at 10:1. Signal gain of the RF/IF

strip inside the IC diminishes as the voltage on CAGC

decreases. Further discussion on setting the attack time

constant is found in “Application Note TBD. Modification of

the attack/decay ratio is possible by adding resistance from

CAGC pin either to VDDBB or VSSBB, as desired. Both the

Push and Pull current sources are disabled during SHUT,

which holds the voltage across CAGC, and improves

recovery time in duty-cycled applications. To further

improve duty cycle recovery, both Push and Pull currents

are increased by 45X for approximately 10msec after

release of SHUT. This allows rapid recovery of any voltage

droop on CAGC while in SHUT.

Figure 1 ANT Pin

4. DO and WAKEB Output Pins

1. ANT Pin

The ANT pin is internally AC-coupled via a 3pF capacitor, to

an RF N-channel MOSFET, as shown in Figure 1.

Impedance on this pin to VSS is quite high at low

frequencies, and decreases as frequency increases. In the

UHF frequency range, the device input can be modeled as

6.3kΩ in parallel with 2pF (pin capacitance) shunt to

VSSRF.

2. CTH Pin

Figure 2 illustrates the CTH pin interface circuit. CTH pin is

driven from a P-channel MOSFET source-follower biased

with approximately 10µA of bias current. Transmission

gates TG1 and TG2 isolate the 6.9pF capacitor. Internal

control signals PHI1/PHI2 are related in a manner such that

the impedance across the transmission gates looks like a

“resistance” of 90kohms. The DC potential on the CTH pin

is approximately 1.6V.

The output stage for the signals DO and WAKEB is shown

in Figure 4. The output is a 35µA push-35µA pull, switched

current stage. Such an output stage is capable of driving

CMOS-type loads.

An external buffer-driver is

recommended for driving high capacitance loads.

5. REFOSC Pin

The REFOSC input circuit is shown in Figure 5. Input

impedance is quite high (200kΩ). This is a Colpitts

oscillator, with internal 30pF capacitors. This input is

intended to work with standard ceramic resonators,

connected from this pin to VSSBB, although a crystal may

be used instead, where greater frequency accuracy is

required. The resonators should not contain integral

capacitors, since these capacitors are contained inside the

IC. Externally applied signals should be AC-coupled,

amplitude limited to approximately 0.5Vpp. The nominal

DC bias voltage on this pin is 1.4V.

6. Control Inputs (SEL0, SEL1, SWEN, SHUT)

Control input circuitry is shown in Figures 6a and 6b. The

standard input is a logic inverter constructed with minimum

geometry MOSFETs (Q2, Q3). P-channel MOSFET Q1 is a

large channel length device which functions essentially as a

“weak” pullup to VDDBB. Typical pullup current is 5µA,

leading to an impedance to the VDDBB supply of typically

1MΩ.

October 1999

11

MICRF003

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